Do not promote position sticky or fixed elements unless they move with scroll.

third_party/WebKit/Source/core/paint/PaintLayer.h

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Intel Corporation. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
@@ skipping 599 matching lines @@
 
   void updateFilters(const ComputedStyle* oldStyle,
                      const ComputedStyle& newStyle);
   void updateClipPath(const ComputedStyle* oldStyle,
                       const ComputedStyle& newStyle);
 
   Node* enclosingNode() const;
 
   bool isInTopLayer() const;
 
-  bool sticksToViewport() const;
+  bool sticksToScroller() const;

[chrishtr, 2017/03/16 00:24:04]

Please add comments documenting these.

[flackr, 2017/03/20 21:04:59]

Done.

+  bool fixedToViewport() const;
   bool scrollsWithRespectTo(const PaintLayer*) const;
 
   void addLayerHitTestRects(LayerHitTestRects&) const;
 
   // Compute rects only for this layer
   void computeSelfHitTestRects(LayerHitTestRects&) const;
 
   // FIXME: This should probably return a ScrollableArea but a lot of internal
   // methods are mistakenly exposed.
   PaintLayerScrollableArea* getScrollableArea() const {
@@ skipping 625 matching lines @@
 
 }  // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 CORE_EXPORT void showLayerTree(const blink::PaintLayer*);
 CORE_EXPORT void showLayerTree(const blink::LayoutObject*);
 #endif
 
 #endif  // Layer_h